Downhole pressure and temperature measuring gauges are utilized in well bore operations to measure temperature and pressure conditions in the well bore. As disclosed and discussed in U.S. Pat. No. 4,628,995, a gauge carrier can be attached to a packer and a perforating device is lowered through the tubing to a location below the packer to perforate the well bore below the set packer. Alternatively, the perforating device can be attached to a pipe below the packer and run in the well bore while attached to the packer. In the '995 patent, the gauge carrier accommodates three gauges located around the circumference of the gauge carrier and each gauge carrier is shock mounted against longitudinal vibration by belleville springs. This system has been highly successful in isolating the shock sensitive pressure gauges from the longitudinal shock forces produced in the well bore during running in the well bore and from detonation of perforating guns when in the well bore.
One of the reasons that pressure gauges are sensitive to shock forces is that typically a quartz transducer is utilized for high accuracy measurement and these transducers are particularly sensitive to shock impact forces. Obviously, the well operator who has gone to the trouble of performing well operations to perforate and to obtain contemporaneous pressure measurements expects the pressure gauge to function. If the gauge malfunctions because the shock forces of the perforator damage the pressure gauge, be it quartz or otherwise, the entire operation is of no value.
The gauge carrier in '995 patent cannot be utilized in deep, small diameter well bores because it's O.D. is too large and it is not intended to be proximate to the perforator means. The smallest O.D. of a pressure gauge is right at 11/4" and thus to reduce the diameter, the gauges (if more than one is used) are stacked in line (end-to-end) and disposed within the central bore of a tubular carrier. Also, in deep, small diameter well bores, the perforating system utilized in conducting a drill stem test has to be lowered on drill pipe and the o.d. of the perforator has a relatively small clearance with respect to the i.d. of the well bore. Perforators used in such tests can utilize as much as a 100 feet of a shot density of four to six shots per foot below a testing packer can thus generate terrifically high shock loads in both longitudinal and lateral directions. Because of the shock loads encountered, the tubular gauge carrier has been spaced a long distance from the perforator to reduce the shock impact effects.
In an effort to get the pressure gauge nearer to the perforator (and thus to obtain more representative pressure measurements) it has been proposed to use pressure gauges coupled end-to-end to one another where the upper pressure gauge is connected to a longitudinal rod member which slidably passes through a spring support plate located in an upper sub member. The spring plate is fixed in position in the sub member. Helical springs are located on the shaft above and below the spring plate. The spring members are compressed to provide a counter balancing spring force so that the attached rod member and pressure gauges can vibrate or move up and down relative to the tool gauge carrier to reduce or dampen the longitudinal shock effects. At spaced intervals along the length of the pressure gauges are radially extending rubber finger centralizers which are short rubber finger elements extending outwardly from the pressure gauge housing to contact the inner wall of the gauge housing. The fingers are four in number and are located at 90.degree. relative to one another and are intended to isolate the gauges from contacting the wall of the gauge carrier in response to lateral shock waves. In this system, the rubber fingers do not provide reliable shock isolation in the lateral direction and the gauges are directly coupled to the rod member so that longitudinal shock effects can cause lateral movement of the gauges in the gauge carrier.
In still another device intended for high shock absorption, the gauges are centralized in a tubular carrier gauge housing by annular rubber or elastomer devices disposed at spaced intervals along the length of the gauges. The annular rubber devices provide for lateral shock absorption universally around the periphery of the gauges. The ends of the gauge carriers are respectively engaged by helical coil springs which are compressed during assembly to exert a centralizing spring force on the gauges within the gauge housing. The springs under compression provide for dampening the shock movement in a longitudinal direction. While this device provides independent lateral and longitudinal isolation of the shock forces from the gauge housing with respect to a gauge carrier, the device is complex and difficult to assemble in the field.
Since the deflection travel of the springs are limited, the assembly has to be matched to the gauge length to obtain the desired force. However, since the assembly is made by inside a housing, it is not possible to check the spring compression. Also where there are sequential tests, it is necessary to completely break down and reassemble the device for each test. Furthermore, if the gauges vary in length, substantial inventory of spare parts is required.